Horizontal Convective Rolls

Horizontal convective rolls (HCRs) are counter-rotating vortex rolls that are nearly aligned with the mean wind of the convective boundary layer. Although horizontal convective rolls have been clearly seen in satellite photographs of cloud streets for the last 30 years, their development is poorly understood due to a lack of observational data. Research has shown these eddies to be significant to the vertical transport of momentum, heat, moisture, and air pollutants within the boundary layer.

HCRs are a manifestation of "thermals" developing in an environment with wind speed increasing with height in the convective boundary layer. In an environment with no vertical wind shear, thermals would produce polygonal areas of convection, which in cross section simply consist of ascending and descending air couplets (Fig. 1).

Figure 1: Classic View of a "Thermal"

When pressure gradients amplify with height (geostrophic wind would increase with height), an air parcel in the ascending branch of the thermal would translate down gradient and would be in a different position relative to the ground. The same would be true in the counterrotating two dimensional thermal adjacent (Fig. 2).

Fig. 2: Evolution of two-dimensional thermals into horizontal convective rolls when the geostrophic wind speed (pressure gradients) increase in magnitude with height in the planetary boundary layer (~1 km depth or so). Red arrows show mean wind direction along the roll axes. Note: this only shows the development of convective rolls in the boundary layer and does NOT show the effect on the HCRs of the deep layer shear.

The alternating counterrotating rolls have horizontal streamwise vorticity in the opposite sense, with vorticity vectors oriented opposite to one another in adjacent rolls. (Fig. 3). Above the boundary layer, if the wind veers sharply above the boundary layer, the horizontal vorticity generated in the cyclonic vorticity HCR is augmented and that in the anticyclonic HCR is suppressed. However, this effect will not occur if the HCRs occur in a boundary layer topped by a capping inversion (since the wind veer with height occurs above the capping inversion).

Fig. 3: Using right hand screw rule, the orientation of the streamwise vorticity vectors is shown for each convective roll. Red indicates cyclonic streamwise vorticity and blue indicates anticyclonic streamwise vorticity. The usual profile of vertical wind shear above the boundary layer should increase the cyclonic vorticity but decrease the anticyclonic vorticity. This implies that in the usual pattern, HCRs "ingested" into thunderstorm updrafts will preferentially have cyclonic vorticity and contribute to cyclonic storm rotation.

If the HCRs occur in relatively moist air, then cloud development will occur in the ascending branch in each HCR, and well-demarked cloud streets will be visible on satellite imagery, but be aware that the actual horizontal vorticity vectors are aligned parallel to the cloud bands but in the clear air. The horizontal vorticity vectors would be alternating oriented (in Fig. 3) northeast in the cyclonic rolls and southwest in the anticyclonic rolls.

HCRs can be associated with deep convection since the ascending branch can be considered a source of parcel lift. If the LFC is low enough, deep convection can initiate on HCRs. Another way to view this is that HCRs will exist in the early morning in the Loaded Gun Sounding environment. As the environmental air destablizes during the afternoon, the convective boundary layer will deepen while the LFC lowers. If the LFC lowers to the top of the Convective Boundary Layer, then deep moist convection will initiate.